1. Field of the Invention
The present invention relates to the suspension of a vehicle such as an automobile, and more particularly, to a control of the damping performance of the vehicle suspension.
2. Description of the Prior Art
The suspension of a vehicle such as an automobile generally comprises a parallel combination of a spring and a damper to support a vehicle body on a vehicle wheel. In such a suspension mechanism, the term "shock absorber" is more generally used instead of the "damper". However, since the damper herein described operates as more than a mere shock absorbing means, the device is herein particularly called "damper". As well known in the art, when the damping coefficient of the suspension damper is low, vertical oscillation of the vehicle wheel is generally well damped or absorbed by the damper not to be transmitted to the vehicle body, but the suspension lacks in the firmness, and the stability of the vehicle is lost, particularly when the vehicle is subjected to a low frequency vertical force from the road surface.
In order to more widely adapt the suspension damper to various vertical oscillations of the vehicle wheel due to variations of road surface and driving speed, variable dampers have been developed in various principles and designs such that the damping coefficient is varied according to a control signal.
When the suspension damper is available as such a variable damper, the performance of the vehicle suspension can be more effectively improved by controlling the suspension according to the sky hook theory which itself is already well known in the art. An art of further improving the suspension performance of the vehicle based upon the sky hook theory is described in U.S. Pat. No. 5,324,069 or corresponding European Patent Laid-open Publication 0566091A1 assigned to the same assignee as the present application. According to the sky hook theory, the actual damper mounted between the vehicle wheel and the vehicle body in parallel with a suspension spring can perform as if it were mounted between the vehicle body and a stationary overhead support, if the damping coefficient C of the actual damper is controlled to be a product of the damping coefficient Cs of such a fictitious damper and a ratio of absolute vertical velocity Vb of the vehicle body to relative vertical velocity Vs of the vehicle body against the vehicle wheel, i.e. stroking velocity of the damper, as follows: EQU C=Cs.times.Vb/Vs
Therefore, if the damping coefficient of the actual damper is variably controllable according to the ratio Vb/Vs (provided that Cs remains constant), the damping performance of the suspension will be much improved such that the vehicle body is damped against its vertical oscillation directly from a stationary overhead support via a damper as not affected by vertical movement of the vehicle wheel.
However, with respect to the adaptation of the damper of the vehicle suspension, there are still other factors. Although the damping performance of the suspension damper is initially adjusted based upon an estimation of certain operating conditions of the vehicle, the actual operating conditions are not always as estimated. Further, although the damping performance of each damper is nominally of a predetermined design, there is an unavoidable difference in the actual damping performance thereof according to a manufacturing allowance, and still further, the damping performance is affected by aging thereof.
Still further, more essentially, the damping performance of the vehicle suspension is affected by the load of the vehicle, i.e. the mass of the driver and the passenger or passengers, when there is or are. The damping force of the vehicle suspension damper acts against the dynamic force of the vehicle body which varies according to the total mass of the vehicle body including its load.
Therefore, it is desirable that the damping performance of the vehicle suspension damper is automatically adjusted time to time according to the operating conditions of the vehicle, or operation environment in more general concept.
On the other hand, if the fictious sky hook damper does not merely remain as an invariable damper while the damping coefficient C of the actual damper is varied according to Vb/Vs but if the damping coefficient Cs of the sky hook damper is also varied to compensate for the mass change of the vehicle body and other casual changes of external and internal operation environment of the vehicle, further improvements of the vehicle suspension performance will be expected.